A standby battery bank is an electrical system which comprises enough individual cells connected in series or series parallel to supply the required voltage and current for its designed load. It is known that, after long periods of time on standby charge, when the battery bank is finally called upon to provide the energy for the load, the battery bank may not be capable of performing as designed. The most common cause of battery failure is due to increased internal resistance of the cell itself, such as from chemistry degradation, or due to the resistance of the connections to it. With respect to the connections, corrosion often develops between bus bars and terminals interconnecting the cells as a result of the presence of strong acid vapors. When load is applied, the resulting high current causes a voltage drop across all the resistances which reduces the output voltage of the battery bank.
For example, if the connection resistance becomes 0.01 .OMEGA. and 1000 amperes are drawn, then a 10 V drop occurs, and about 10 KW of heat is generated thus causing the cell to overheat. On a 48V system then, unacceptably, only 38V remain available for use. Degradation in cell chemistry can be even more serious. If, for example, internal resistance increases to 1 .OMEGA., then a 48 V system is unable to sustain a 1000 ampere draw at all and the output voltage is caused to fall to zero.
Accordingly, to avoid suffering a system failure, just when the standby batteries are most needed, several methods currently exist to identify a cell having high resistance.
One such method involves removing the entire bank from standby duty and replacing it with a temporary substitute. A known load, similarly sized as a substitute for the designed load, is placed across the battery bank and the voltage across each cell is measured. Ohm's law (V=IR) is then used to calculate the resistance between each cell. This calculated resistance is the sum of the resistance of the connections and the internal resistance of the cell itself. This process is expensive, requires installation of heavy wires across dangerously high voltages in a corrosive environment, and are prone to cause problems with the battery bank.
Other methods include imposing a large load across the entire battery bank while it continues to charge. Individual cell voltages are then multiplexed to a single measurement instrument. For example, in U.S. Pat. No. 5,705,929 to May 23, 1995 to Caravello et al., a test is disclosed for applying a large, high-current-capable load across the entire battery bank. Long wires extend from each cell to be multiplexed onto a measurement instrument located centrally to the battery bank. The Carvello approach requires many long wires and the installation of large, expensive, heat-generating resistors and power switches. Further, due to the large drain imposed and maintained on the battery bank, it is significantly discharged during the test.
In another typical characteristic of standby battery banks, because a plurality of individual cells are connected in series, each cell receives the same amount of charging current. Therefore, the float charging current is typically set to a value required by the most needy individual cell. As a result, some cells tend to be over-charged.
Proposals to correct this over-charging have been made. In U.S. Pat. No. 4,614,905 filed Sep. 30, 1986 by Petersson et al, over-charging is controlled by imposing a varying load across each cell in the bank. Petersson discloses connecting a transistor across each cell so that a small amount of current is dynamically bypassed around each cell, dependent on the cell's voltage--its state of charge. Unfortunately, this method does not contemplate measuring the cell's overall resistance and further, it imposes a constant float current which, if flowing for long extended periods, tends to degrade the life of the cell.
As discussed above, applicants are not aware of prior art which adequately address the problems in reliably measuring the resistances of individual cells of a battery bank. Therefore, there is a demonstrated need for means which allow individual cell resistances to be frequently and safely measured, while the battery bank is still in the system, in use, or while charging. Preferably, the resistance measurement is accomplished without discharging the cells, and further provides means to equalize individual cells without imposing a constant current for extended periods of time.